<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        li {
            display: inline;
        }

        .box {
            width: 100px;
            /* height: 100px; */
            border: 1px solid #000;
            /* overflow: hidden; */
        }

        .item {
            width: 50px;
            height: 50px;
            border: 1px solid blue;
            margin-bottom: 10px;
            /* float: left; */
        }

        .item-2 {
            width: 50px;
            height:50px;
            border: 1px solid red;
            margin-top: 10px;
            overflow: hidden;
        }

        .box::after {
            content: "";
            display: block;
            height: 0;
            clear: both
        }

        /* .clear {
            clear: both;
        } */
    </style>
</head>

<body>
    <div>this is one</div>
    <div>this is one</div>
    <div>this is one</div>
    <ul>
        <li>this is one</li>
        <li>this is two</li>
        <li>this is three</li>
    </ul>
    <div class="box">
        <div class="item"></div>
        <div class="item-2"></div>
        <!-- <div class="clear"></div> -->
    </div>
</body>

</html>